Block 2 - PLC Ladder Logic - SAMA Drawings

Question 1

What is a PLC?

Answer 1

A PLC, or Programmable Logic Controller, is a digital computer used to automate industrial processes.

PLCs are often combined with sensors, actuators, and HMIs to create automated systems and are utilized in processes like assembly lines and HVAC systems.

Question 2

How are PLCs programmed?

Answer 2

PLCs are programmed using specialized software and a programming language, typically ladder logic or a text-based language.

Each PLC brand may have its proprietary programming software.

Question 3

What is PLC Ladder Logic?

Answer 3

PLC ladder logic is a graphical programming language that resembles a ladder, with vertical rails and horizontal rungs.

It visually represents the control process.

Question 4

How does ladder logic work?

Answer 4

In ladder logic, each rung represents a specific action or operation the PLC will perform, arranged logically from top to bottom.

Each rung contains instructions that guide the PLC’s actions.

Question 5

What are contacts and coils in ladder logic?

Answer 5

Contacts represent inputs or conditions that must be met for execution, while coils represent outputs or actions that occur upon execution.

They are used to create logical operations like AND, OR, and NOT.

Question 6

What are the two main categories of PLC ladder logic inputs?

Answer 6

Digital inputs and analog inputs.

Digital inputs are binary signals, while analog inputs are continuous signals.

Question 7

What are digital inputs in PLCs?

Answer 7

Digital inputs are binary signals that are either ‘on’ or ‘off’ used to detect the presence or absence of a signal.

They commonly sense the state of switches and sensors.

Question 8

What are analog inputs in PLCs?

Answer 8

Analog inputs are continuous signals with values within a specified range, used to measure and control parameters like temperature and pressure.

They can also measure flow rate and voltage.

Question 9

What are special inputs in PLCs?

Answer 9

Special inputs include high-speed counter inputs for pulse counting and encoder inputs for measuring position and speed.

These are represented in ladder logic diagrams using dedicated function blocks.

Question 10

What are PLC ladder logic outputs?

Answer 10

PLC ladder logic outputs are signals that control the behavior of devices connected to the PLC system, categorized into digital and analog outputs.

Outputs are also represented in ladder logic diagrams.

Question 11

What are digital outputs in PLCs?

Answer 11

Digital outputs are binary signals, either ‘on’ or ‘off’, used to control the state of switches and relays.

They are represented in ladder logic diagrams using output coils.

Question 12

What are analog outputs in PLCs?

Answer 12

Analog outputs are continuous signals used to control parameters such as temperature and pressure.

They are illustrated in ladder logic diagrams using function blocks like DACs.

Question 13

What is the role of PLC outputs?

Answer 13

PLC outputs switch electrical loads like lights and motors but cannot independently provide power.

A separate power source is required to operate these loads.

Question 14

What does a PID process loop controller do?

Answer 14

A PID process loop controller generates an output to apply corrective effort to a process, driving a measurable process variable toward a desired set-point value.

Question 15

What is the function of an actuator in a PID loop?

Answer 15

An actuator influences the process based on the control signal from the PID controller.

It works alongside a sensor that measures the results.

Question 16

What is the scale factor in a PID loop?

Answer 16

The scale factor maps the process value to the output value, typically ranging from 0 to 100%.

It is essential for matching temperature changes with percentage span changes.

Question 17

Fill in the blank: The general mapping equation for PID is output/process value = _______.

Answer 17

Scale Factor

Question 18

How is the scale factor calculated in a PID loop?

Answer 18

The scale factor is calculated using the formula: output/process value = Scale Factor.

This involves using full scale values for the output and process.

Question 19

What happens to the control system’s response speed with proportional gain adjustments?

Answer 19

Increases response speed, reduces rise time, increases overshoot, and reduces steady-state error.

Question 20

What is a SAMA Drawing?

Answer 20

Functional control diagrams for the power industry based on symbols and conventions developed by the Scientific Apparatus Makers Association.

SAMA diagrams describe and document control strategies and systems for industrial and utility boiler applications.

Question 21

What are SAMA diagrams used for?

Answer 21

To describe and document control strategies and systems for industrial and utility boiler applications.

They are sometimes called ‘control functional diagrams’.

Question 22

What types of symbols are included in SAMA diagrams?

Answer 22

• Enclosure symbols
• Signal continuation symbols
• Signal processing symbols

These symbols represent different elements or functions of the control system.

Question 23

What does an enclosure symbol in a SAMA diagram represent?

Answer 23

Different elements or functions of the control system.

Question 24

What do continuation symbols do in SAMA diagrams?

Answer 24

Connect enclosure symbols within the diagrams.

Question 25

What is the purpose of signal processing symbols in SAMA diagrams?

Answer 25

Describe functions within each enclosure.

Question 26

True or False: Strict adherence to the SAMA standard is common.

Answer 26

False.

Variations in symbol use are frequent.

Question 27

What does PID stand for in control systems?

Answer 27

Proportional plus Integral plus Derivative.

Question 28

How many inputs does a PID controller generally have?

Answer 28

Two inputs: the process variable (PV) and the setpoint (SP) value.

Question 29

What does the PID controller calculate?

Answer 29

The control error (Δ) between the process variable and the setpoint.

Question 30

Fill in the blank: The classic SAMA symbol for a PID controller uses standard mathematical symbols for these functions: _____, _____, and _____.

Answer 30

P, I, D.

Question 31

What does the diamond shape represent in a basic single loop control diagram?

Answer 31

Manual functions performed by the operator.

Question 32

What is the simplest form of drum level control?

Answer 32

Single-element drum level control.

Question 33

What two elements constitute the two-element drum level control method?

Answer 33

• Level element (drum level)
• Steam flow element.

Question 34

What is the purpose of the three-element drum level control?

Answer 34

To provide tighter drum level control under fluctuating steam loads.

Question 35

What risks are associated with low and high drum levels in boiler operation?

Answer 35

• Low drum level risks exposing boiler tubes to heat stress and damage.
• High drum level risks water carryover into the steam header, potentially causing corrosion and damage to steam turbines.

Question 36

What does the summing function (Σ) do in the three-element drum level control strategy?

Answer 36

Combines the feedwater component and cascade component of the control strategy.

Question 37

Fill in the blank: SAMA diagrams provide a detailed and standardized way to represent _____ and _____ in the power industry.

Answer 37

Complex control strategies, systems.